Projector logs numDocsPending stats every 1min. To that extent, is spawns the go-routines to fetch seqnos from KV node. One go-routine queries ns_server every 10sec to validate the presence of a bucket in the cluster and clear the book-keeping if the bucket wa to be deleted.
In cases where there are large number of buckets, periodically querying all the bucket endpoints is adding significant load on ns_server i.e. in a 30 bucket setup, it would be 30*6 = 180 requests per min. These requests would be to "pools/default/buckets/<bucket_name>?uuid=<>" endpoint which is a relatively costly call.